Interventional Radiology (IR) is a rapidly growing branch of medicine which uses medical imaging systems such as Computer Tomography, Magnetic Resonance, X-ray and fluoroscopy for therapeutic as well as diagnostic purposes. In IR, a physician wishes to guide a catheter to a remote site within the body for the purpose of making measurements, retrieving samples (biopsy), effecting therapeutic actions (for example, angioplasty of an artery wall), or blocking of an artery feeding a tumor (embolization). The catheter is a thin tube (2-6 mm diameter) on the order of 1 meter long which contains a number of interior passages (depending on design) and which is guided by a flexible, removeable, radio opaque internal guide wire.
The circulatory system can be thought of as a tree-like structure fanning out from a central arterial tube at the heart. The diameter of the main aorta is on the order of 2-4 cm. Beginning with the bifurcation of the main aorta, each subsequent branch forks or bifurcates into two or more smaller diameter branches Eventually at the cellular level the arterial diameter narrows to that of an individual red blood cell. The veinous system is responsible for collecting the blood cells from the capillaries for eventual return to the heart. The geometry of the veinous network is the inverse of the arterial system, with small tubes merging to form larger tubes which merge to form still larger tubes. While there is considerable similarity in topology and geometry among individuals at a gross level, at a detailed level the vascular system has a complex topology with a tortuous three-dimensional geometry which is unique to each individual.
The goal of an IR procedure is to deliver the working end of a catheter to an internal site within the body of the patient. The vascular system is used to physically contain the catheter and to act as the conduit along which the catheter progresses. Access to the vascular system is via a puncture with a tubular sleeve which remains in place during the procedure. The catheter and guide wire are threaded through the sleeve.
The fluoroscope (a 30 Hz X-ray machine) is the primary tool used by the physician to help guide the catheter. In many cases, his knowledge of general vascular anatomy, his experience, and the "in-process" fluoroscope images provide sufficient information to enable the physician to reach the target site. Typically, fluoroscope images are formed at a rate of 30 per second and displayed on a TV-like monitor. In order to see the local structure of the vascular system in the vicinity of the catheter tip more clearly, the physician injects a radio opaque dye into the catheter As the dye flows from the end of the catheter into the bloodstream, it temporarily renders the vein or arter opaque, causing the vessel to become a silhouette on the fluoroscope monitor. Depending on the amount of dye introduced, the rate at which it is introduced, and the blood flow rate, the opacification lasts from 2-5 seconds. As new blood arrives, the dye is diluted and swept away and the opacification gradually fades. This procedure can be very uncomfortable for the patient since the dye causes a burning sensation as it enters the bloodstream.
The sites that are particularly well suited to IR-based treatments lie further along the vascular "highway". For these situations the physician acquires what is called a "roadmap". For example, if the goal is to thread a catheter to a site deep within the kidney, the physician would first guide the catheter tip to the vicinity of the main artery feeding the kidney. A large enough amount of dye is introduced to opacify most or all arteries within the organ. As the dye is introduced, the fluoroscope is turned on and runs during the next 10-20 seconds. The images are stored for replay after the dye injection. After dye injection, the images are reviewed and the most representative image is transferred to a display monitor to be used as a two-dimensional reference image for the actual therapeutic procedure. Unfortunately the reference image or roadmap image is only a two-dimensional projection of the complex three-dimensional anatomy of the patient. Many systems in use allow the physician to subtract this roadmap from the live fluoroscopic image; in this mode the roadmap image acts as a mask which removes the background, non-vascular tissue (usually bones) and highlights the opacified vessels and catheter guide wire in the subtracted output image. Since the bones in the image remain constant in both the raw image and the roadmap image, when the images are subtracted, the bones disappear, the catheter guide wire remains black, and the opacified vessels become white. It usually takes multiple dye injections for the physician to reach the desired body location:
The next step is to guide the catheter ("avigate") to the desired site within the organ. The physician does this by viewing the fluoroscope image and referring to the two-dimensional roadmap image. He advances the catheter tip slightly, injects a small amount of dye into the catheter, observes the locally opacified arteries, compares the real-time image to the static roadmap image and determine if he is "on-course". If the vascular network lies largely in a plane that is perpendicular to the imaging axis, then the navigation is not difficult. Unfortunately, most of the vasculature at the organ level is of small diameter and twists and turns in a complicated way in three dimensions. In these cases, the two-dimensional roadmap and the two-dimensional real-time fluoroscope images can be very confusing. Choosing the correct pathway can become a time consuming trial and error procedure as the physician injects dye and attempts to interpret the imagery. In many cases, the only recourse is to physically rotate the imaging axis with respect to the patient to gain a view from another perspective. This is called "angulating" the fluoroscope. In many cases, it may be necessary to angulate and inject a number of times as the physician attempts to "make sense" of the projected roadmap image.
What is needed is a three-dimensional roadmapping and real-time guidance system that is analogous to the current two-dimensional system.